Manufacturers are continually striving to improve the efficiency of their manufacturing operations to gain or maintain an advantage over their competition. Properly controlling the amount of work released into a manufacturing system and the dispatching work to work centers within the system generally are important for maximizing the throughput of the system. These control functions, however, may be affected by many variables in the system, especially in a complicated manufacturing process. For example, modern semiconductor integrated circuit manufacturing may involve hundreds of steps, with multiple products being manufactured at the same time. In addition, various work stages may take different amounts of time to complete, and some stages may operate on individual wafer lots, while others may operate on several wafer lots at the same time in a batch mode.
To improve control of the manufacturing process, various methods of controlling the release of work have been implemented in manufacturing operations such as semiconductor fabrication facilities (fabs). Traditional work release policies generally may be classified into two different types of approaches, push systems such as material requirements planning (MRP), and pull systems such as constant work-in-progress (CONWIP). There are, however, drawbacks to each approach.
As a push type system, MRP generally releases lots into a manufacturing facility according to a demand plan. MRP determines what assemblies must be built and what materials must be procured in order to build a unit of work by a certain date. MRP queries the bill of materials and inventory databases to derive the necessary elements. One disadvantage of the MRP approach, however, is that it does not consider plant conditions in determining the work release plan. Because of limitations in the plant, an MRP-generated schedule may get so far ahead of production that it can overwhelm the manufacturing plant with work-in-progress (WIP).
In contrast, as a pull type system, CONWIP generally releases lots according to a pre-defined work in progress cap or limit. Because CONWIP considers the manufacturing facility's conditions, it generally avoids the MRP problem of excessive WIP. CONWIP, however, generally considers the facility status in only a rudimentary manner, and is not sufficiently detailed to account for dynamic fab conditions.
Another potential problem with CONWIP is that it generally does not consider the capacity impact of complex product mixes or customer requests. Critical ratio (C/R) may be defined as [Master Production Schedule (MPS) date−Work Start date]/[Forecast cycle time (C/T)]. For a semiconductor fab, Work Start is Wafer Start (W/S). Forecast C/T may be determined using historical fab data. C/R can provide a measure of the urgency of each lot for meeting on-time delivery. When C/R>1, a lot is ahead of schedule; when C/R<1, a lot is behind schedule. C/R may measure demand through the MPS value and supply through the forecast C/T. If a wafer only starts when C/R=1 and forecast C/T uses a fixed and maintained number, then this is generally the same as the MRP approach. C/R is generally only meaningful inside the same product group, however, because it cannot objectively compare products that belong to different capacity constraints.
In some situations, production control (PC) planners may rely on their own personal experience to determine a work release plan. For example, they may attempt to estimate a wafer start quantity in an informal manner based on some combination of the monthly MPS target, the fab front-end machine conditions (M/C), WIP conditions, and customer requests. For example, for production lots, PC account planners book lots on booking files. Then PC fab planners schedule new-booked lots on the Manufacturing Execution System (MES), checking whether part status is ready or not. Then PC fab planners reschedule all lots according to critical ratio and fab line balance.
For pilot lots and probe card wafers, PC account planners book lots on booking files according to Engineering Notice/Order Management. Then PC pilot planners schedule new-booked lots on the MES, checking whether mask status is ready or not. Finally, for engineering lots/loop test, Engineering Integration engineers issue an application form, then PC pilot planners schedule lots on the MES. Obviously, the success of such an experience-based approach depends heavily on the level of experience and skill of the individual, and a wide range of results may be realized when relying on personal experience as a guide for work release.
Currently, there does not exist a systematic approach for determining the proper amount of work to release to a production line without the limitations of the approaches described above.